Cyclic AMP-induced Conformational Changes in Mycobacterial Protein Acetyltransferases

The activities of a number of proteins are regulated by the binding of cAMP and cGMP to cyclic nucleotide binding (CNB) domains that are found associated with one or more effector domains with diverse functions. Although the conserved architecture of CNB domains has been extensively studied by x-ray crystallography, the key to unraveling the mechanisms of cAMP action has been protein dynamics analyses. Recently, we have identified a novel cAMP-binding protein from mycobacteria, where cAMP regulates the activity of an associated protein acetyltransferase domain. In the current study, we have monitored the conformational changes that occur upon cAMP binding to the CNB domain in these proteins, using a combination of bioluminescence resonance energy transfer and amide hydrogen/deuterium exchange mass spectrometry. Coupled with mutational analyses, our studies reveal the critical role of the linker region (positioned between the CNB domain and the acetyltransferase domain) in allosteric coupling of cAMP binding to activation of acetyltransferase catalysis. Importantly, major differences in conformational change upon cAMP binding were accompanied by stabilization of the CNB and linker domain alone. This is in contrast to other cAMP-binding proteins, where cyclic nucleotide binding has been shown to involve intricate and parallel allosteric relays. Finally, this powerful convergence of results from bioluminescence resonance energy transfer and hydrogen/deuterium exchange mass spectrometry reaffirms the power of solution biophysical tools in unraveling mechanistic bases of regulation of proteins in the absence of high resolution structural information.

Dual emissive borane-BODIPY dyads: molecular conformation control over electronic properties and fluorescence response towards fluoride ions

Facile synthesis of two new dimesitylboryl appended BODIPYs is reported. The two dyads have similar fluorescent chromophores but differ in their molecular conformations. They exhibit dual fluorescence, intramolecular energy transfer between boryl and BODIPY chromophores and different fluorescence responses (emission enhancement and quenching) upon fluoride binding.

Ultrafast photoinduced enhancement of nonlinear optical response in 15-atom gold clusters on indium tin oxide conducting film

We show that the third order optical nonlinearity of 15-atom gold clusters is significantly enhanced when in contact with indium tin oxide (ITO) conducting film. Open and close aperture z-scan experiments together with non-degenerate pump-probe differential transmission experiments were done using 80 fs laser pulses centered at 395 nm and 790 nm on gold clusters encased inside cyclodextrin cavities. We show that two photon absorption coefficient is enhanced by an order of magnitude as compared to that when the clusters are on pristine glass plate. The enhancement for the nonlinear optical refraction coefficient is similar to 3 times. The photo-induced excited state absorption using pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also show an enhancement by an order of magnitude. These results attributed to the excited state energy transfer in the coupled gold cluster-ITO system are different from the enhancement seen so far in charge donor-acceptor complexes and nanoparticle-conjugate polymer composites.

Tuning the parameters to establish Quenching and enhancement regimes in hybrid Gold nanoparticles and CdSe Quantum dots monolayer

The multi-component nanomaterials combine the individual properties and give rise to emergent phenomenon. Optical excitations in such hybrid nonmaterial's ( for example Exciton in semiconductor quantum dots and Plasmon in Metal nanomaterials) undergo strong weak electromagnetic coupling. Such exciton-plasmon interactions allow design of absorption and emission properties, control of nanoscale energy-transfer processes, and creation of new excitations in the strong coupling regime.This Exciton plasmon interaction in hybrid nanomaterial can lead to both enhancement in the emission as well as quenching. In this work we prepared close-packed hybrid monolayer of thiol capped CdSe and gold nanoparticles. They exhibit both the Quenching and enhancements the in PL emission.The systematic variance of PL from such hybrid nanomaterials monolayer is studied by tuning the Number ratio of Gold per Quantum dots, the surface density of QDs and the spectral overlap of emission spectrum of QD and absorption spectrum of Gold nanoparticles. Role of Localized surface Plasmon which not only leads to quenching but strong enhancements as well, is explored.

Dual Binding Site Assisted Chromogenic and Fluorogenic Recognition and Discrimination of Fluoride and Cyanide by a Peripherally Borylated Metalloporphyrin: Overcoming Anion Interference in Organoboron Based Sensors

Peripherally triarylborane decorated porphyrin (2) and its Zn(II) complex (3) have been synthesized. Compound 3 contains of two different Lewis acidic binding sites (Zn(II) and boron center). Unlike all previously known triarylborane based sensors, the optical responses of 3 toward fluoride and cyanide are distinctively different, thus enabling the discrimination of these two interfering anions. Metalloporphyrin 3 shows a multiple channel fluorogenic response toward fluoride and cyanide and also a selective visual colorimetric response toward cyanide. By comparison with model systems and from detailed photophysical studies on 2 and 3, we conclude that the preferential binding of fluoride occurs at the peripheral borane moieties resulting in the cessation of the EET (electronic energy transfer) process from borane to porphyrin core and with negligible negetive cooperative effects. On the other hand, cyanide binding occurs at the Zn(II) core leading to drastic changes in its absorption behavior which can be followed by the naked eye. Such changes are not observed when the boryl substituent is absent (e.g., Zn-TPP and TPP). Compounds 2 and 3 were also found to be capable of extracting fluoride from aqueous medium.

Multichannel-Emissive V-Shaped Boryl-BODIPY Dyads: Synthesis, Structure, and Remarkably Diverse Response toward Fluoride

Three new V-shaped boryl-BODIPY dyads (1-3) were synthesized and structurally characterized. Compounds 1-3 are structurally close molecular siblings differing only in the number of methyl substituents on the BODIPY moiety that were found to play a major role in determining their photophysical behavior. The dyads show rare forms of multiple-channel emission characteristics arising from different extents of electronic energy transfer (EET) processes between the two covalently linked fluorescent chromophores (borane and BODIPY units). Insights into the origin and nature of their emission behavior were gained from comparison with closely related model molecular systems and related photophysical investigations. Because of the presence of the Lewis acidic triarylborane moiety, the dyads function as highly selective and sensitive fluoride sensors with vastly different response behaviors. When fluoride binds to the tricoordinate borane center, dyad 1 shows gradual quenching of its BODIPY-dominated emission due to the ceasing of the (borane to BODIPY) EET process. Dyad 2 shows a ratiometric fluorescence response for fluoride ions. Dyad 3 forms fluoride-induced nanoaggregates that result in fast and effective quenching of its fluorescence intensity just for similar to 0.3 ppm of analyte (i.e., 0.1 equiv 0.26 ppm of fluoride). The small structural alterations in these three structurally close dyads (1 - 3) result in exceptionally versatile and unique photophysical behaviors and remarkably diverse responses toward a single analyte, i.e., fluoride ion.

Organic white-light emitting materials

Full-color emissive organic materials have attracted significant attention in recent years as key components in display and lighting devices based on OLEDs. An ideal white-light emitter demands simultaneous emission of red, green and blue with nearly similar distribution of intensities covering the entire region of visible spectra. However, the design of such white-light emitters is not straightforward. Mixing several emitters is seldom successful owing to the negative effects of intermolecular interactions and energy transfer processes. Nonetheless, these fundamental questions have been addressed in recent times by several research groups of vastly different expertise leading to a considerable progress in the field of organic white-light emitters. The designs cover a large area of the chemistry ranging from frustrated energy transfer to simple protonation or from designed self-assembly to simple mixing of materials. In this review, the concepts and rational approaches underlying the design of white-light emissive organic materials are described. (C) 2014 Elsevier Ltd. All rights reserved.

Electron-Rich Triphenylamine-Based Sensors for Picric Acid Detection

This paper demonstrates the role of solvent in selectivity and sensitivity of a series of electron-rich compounds for the detection of trace amounts of picric acid. Two new electron-rich fluorescent esters (6, 7) containing a triphenylamine backbone as well as their analogous carboxylic acids (8, 9) have been synthesized and characterized. Fluorescent triphenylamine coupled with an ethynyl moiety constitutes p-electron-rich selective and sensitive probes for electron-deficient picric acid (PA). In solution, the high sensitivity of all the sensors toward PA can be attributed to a combined effect of the ground-state charge-transfer complex formation and resonance energy transfer between the sensor and analyte. The acids 8 and 9 also showed enhanced sensitivity for nitroaromatics in the solid state, and their enhanced sensitivity could be attributed to exciton migration due to close proximity of the neighboring acid molecules, as evident from the X-ray diffraction study. The compounds were found to be quite sensitive for the detection of trace amount of nitroaromatics in solution, solid, and contact mode.

Cyclic nucleotide binding and structural changes in the isolated GAF domain of Anabaena adenylyl cyclase, CyaB2

GAF domains are a large family of regulatory domains, and a subset are found associated with enzymes involved in cyclic nucleotide (cNMP) metabolism such as adenylyl cyclases and phosphodiesterases. CyaB2, an adenylyl cyclase from Anabaena, contains two GAF domains in tandem at the N-terminus and an adenylyl cyclase domain at the C-terminus. Cyclic AMP, but not cGMP, binding to the GAF domains of CyaB2 increases the activity of the cyclase domain leading to enhanced synthesis of cAMP. Here we show that the isolated GAFb domain of CyaB2 can bind both cAMP and cGMP, and enhanced specificity for cAMP is observed only when both the GAFa and the GAFb domains are present in tandem(GAFab domain). In silico docking and mutational analysis identified distinct residues important for interaction with either cAMP or cGMP in the GAFb domain. Structural changes associated with ligand binding to the GAF domains could not be detected by bioluminescence resonance energy transfer (BRET) experiments. However, amide hydrogen-deuterium exchange mass spectrometry (HDXMS) experiments provided insights into the structural basis for cAMP-induced allosteric regulation of the GAF domains, and differences in the changes induced by cAMP and cGMP binding to the GAF domain. Thus, our findings could allow the development of molecules that modulate the allosteric regulation by GAF domains present in pharmacologically relevant proteins.

Panchromatic Borane-aza-BODIPY Conjugate: Synthesis, Intriguing Optical Properties, and Selective Fluorescent Sensing of Fluoride Anions

A new triarylborane-aza-BODIPY conjugate is reported. The compound consists of two blue emissive dimesitylarylborane moieties and a near-infrared (NIR) emissive aza-BOIDPY core and shows panchromatic absorption spanning approximately 300-800 nm. DFT computational studies suggest limited electronic communication between the individual fluorophore units. Hence, the partial energy transfer from blue fluorophore triarylborane to NIR chromophore aza-BODIPY unit leads to a broad dual-emissive feature covering a large part of visible and NIR region. Furthermore, the broadband emissive compound can act as a selective sensor for fluoride anion as a result of fluorescence quenching response in both visible and NIR spectral regions.

A hidden symmetry of AdS resonances

Recent investigations have revealed powerful selection rules for resonant energy transfer between modes of nonlinear perturbations in global anti-de Sitter (AdS) space-time. It is likely that these selection rules are due to the highly symmetric nature of the underlying AdS background, though the precise relation has remained unclear. In this article, we demonstrate that the equation satisfied by the scalar field mode functions in AdS(d+1) has a hidden SU(d) symmetry, and explicitly specify the multiplets of this SU(d) symmetry furnished by the mode functions. We also comment on the role this structure might play in explaining the selection rules.

One pot auto-ignition based synthesis of novel Sr2CeO4: Ho3+ nanophosphor for photoluminescent applications

Ho3+ (0.25-7 mol%) doped Sr2CeO4 nanophosphors were synthesized by solution combustion method using urea as fuel. The structural properties of the nanophosphors were investigated by powder X-ray diffraction studies (PXRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. UV-Visible and photoluminescence (PL) spectroscopic techniques were used for analysing the optical properties of the nanoparticles. PXRD and TEM results revealed the formation of Sr2CeO4: Ho3+ nanocrystalline particles with orthorhombic crystal structure. From the UV-Vis studies the optical band gap energy found to decrease from 5.9 to 5.74 eV with increase in dopant concentration. The PL spectra exhibit the broad excitation band from 200 to 400 nm which concurs well with the commercial near UV LED. The PL spectra vary with the dopant content due to energy transfer from the host to the activator. In this present work we demonstrate that color tuning of phosphor can be achieved by merely varying the Ho3+ ions concentration. The CIE and CCT chromaticity coordinates suggests Sr2CeO4: Ho3+ nanophosphors may be potentially applicable as promising single - phased phosphors for lighting applications. (C) 2015 Elsevier B.V. All rights reserved.

Comparative study of Eu3+ -activated LnOCl (Ln=La and Gd) phosphors and their Judd-Ofelt analysis

Eu3+-activated layered LnOCl (Ln=La and Gd) phosphors were synthesized by the conventional solid-state method at relatively low temperature (700 degrees C) and shorter duration of 2 h. The structural parameters were refined by the Rietveld refinement analysis and confirmed by the high resolution transmission electron microscopy (HRTEM). Both the compounds were crystallized in the tetragonal structure with space group P4/nmm (No. 129). The homogeneity of the elements were analyzed by TEM mapping and found to be uniformly distributed. The photoluminescence spectra revealed that the intensity of D-5(0)-> F-7(2) transition (619 nm) was more intense in Eu3+-activated GdOCl compared to LaOCl. This was due to the property of Gd3+ ions to act as an intermediate sublattice to facilitate the energy transfer to Eu3+ ions. Intensity parameters and radiative properties such as transition probabilities, radiative lifetime and branching ratio were calculated using the Judd-Ofelt theory. The CIE color coordinates result revealed that the Eu3+-activated GdOCl (0.641, 0.354) phosphor was close to the commercial red phosphors like, Y2O3:Eu3+ (0.645, 0.347), (Y2OS)-S-2:Eu3+ (0.647, 0.343) and National Television System Committee (NTSC) (0.67, 0.33). The results suggest that the present GdOCl:Eu3+ compound acts as a potential candidate for red phosphor materials.

Incorporation of Cr3+ ions in tuning the magnetic and transport properties of nano zinc ferrite

Cost effective and low temperature synthesis methods namely solution combustion and hydrothermal methods were used to prepare chromium incorporated nanocrystalline zinc ferrites. The effect of incorporation of low concentration Cr3+ ions on the structural, morphological, magnetic and transport properties of the zinc ferrite compounds were investigated. The crystalline nature and size variation with chromium content were valid from powder x-ray diffraction. Particles size and crystallite size variation were valid from scanning electron microscopy and transmission electron microscopy respectively. With the increase in chromium incorporation, the crystallite and particles sizes were decreased. Fourier transform infrared spectroscopy (FTIR) studies confirmed the presence of strong metal-oxygen bonds. The elastic properties of the materials in both the methods were estimated by FTIR studies. Magnetic properties namely saturation magentization, remanent magnetization and coercivity values were decreased with increase in Cr3+ ions concentration. The dielectric properties of the samples decreased with increase in the Cr3+ ions. The dielectric constant was observed to be of the order of 10(6) at low frequency and almost 1 at higher frequency range. The activation energy estimated using Arrhenius plots was of the order of 0.182 eV and 0.368 eV respectively for the compounds prepared by solution combustion and hydrothermal methods. The emission spectra of the samples excited at 344 nm were reported using photoluminescence (PL) spectroscopy. Further, the approximate energy band gap(E-g) was estimated from PL studies. The E-g of the materials were lie in the range of 2.11-1.98 eV. (C) 2015 Elsevier B.V. All rights reserved.

Folding of Protein L with Implications for Collapse in the Denatured State Ensemble

A fundamental question in protein folding is whether the coil to globule collapse transition occurs during the initial stages of folding (burst phase) or simultaneously with the protein folding transition. Single molecule fluorescence resonance energy transfer (FRET) and small-angle X-ray scattering (SAXS) experiments disagree on whether Protein L collapse transition occurs during the burst phase of folding. We study Protein L folding using a coarse-grained model and molecular dynamics simulations. The collapse transition in Protein L is found to be concomitant with the folding transition. In the burst phase of folding, we find that FRET experiments overestimate radius of gyration, R-g, of the protein due to the application of Gaussian polymer chain end-to-end distribution to extract R-g from the FRET efficiency. FRET experiments estimate approximate to 6 angstrom decrease in R-g when the actual decrease is approximate to 3 angstrom on guanidinium chloride denaturant dilution from 7.5 to 1 M, thereby suggesting pronounced compaction in the protein dimensions in the burst phase. The approximate to 3 angstrom decrease is close to the statistical uncertainties of the R-g data measured from SAXS experiments, which suggest no compaction, leading to a disagreement with the FRET experiments. The transition-state ensemble (TSE) structures in Protein L folding are globular and extensive in agreement with the Psi-analysis experiments. The results support the hypothesis that the TSE of single domain proteins depends on protein topology and is not stabilized by local interactions alone.